grepview: Fix search in project root with trailing slash

Summary:
Search in files did not work if you choose to search in the current
project and then append a "/" to the location (e.g.
"/home/user/kdevplatform/").

Reviewers: kfunk, mwolff

Reviewed By: kfunk

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4774